Light not going through the glass in the window
You have 3 options.


Let the room Recieve Transperent
Use Cycles render
Let the Sun light cast no shadows


I believe the first one is most helpful.

1. Let the room Recieve Transperent

Change the material applied to the room. (Not the window)

If you scroll down, you can find the Shadow panel. Tick the Recieve transperent.

This works with the default blender render.

2. Use Cycles render

Cycles is a physic based render and let the light penetrate through the window.
For links, check the comment on the question.

3. Let the Sun light cast no shadows

I believe this is not your required solution but just in case this was.
The ray will IGNORE the ceiling.

Go to the light setting of the sunlight and Switch from Ray shadow to No shadow. (at the Shadow panel)

What I think is best

Combining 1 and 3 might be a neat solution.
Make two sunlights and let one light ray shadows, the other, none. This will lighten up the entire room while especially lighting up the wall on the opposite side of the window.